1. Field of the Invention
This invention is related to high voltage electrical insulators intended for operation at voltages of greater than 15 kilovolts.
2. Background Information
High voltage transmission lines have historically been insulated with porcelain and glass insulators. In order to operate at the reliability level required, these insulators are designed to operate at a low electrical stress level. For use in clean atmospheres the stress level is generally about 1 kV per inch. In areas where the insulator is subjected to contamination, as along a sea coast or in an industrial area, the stress level is ordinarily on the order of 0.5 kV per inch or less, resulting in a large and bulky insulator. These insulators are very heavy and still develop high level leakage current and dry band arcing, which leads to flashover. In practice, utilities often carry out extensive insulator maintenance such as washing and greasing, or coating with a room temperature curing silicone rubber, of the insulators at regular intervals of time. In some severe applications, utilities have gone to the use of resistance grading of the insulator to give heated insulators to avoid excessive leakage currents.
Other types of insulators have been proposed for these types of uses, such as those below as described in various patents and scientific literature.
United Kingdom Pat. No. 915,052, published Jan. 9, 1963, teaches an electrical insulator comprising a rod or tube of resin-bonded glass fiber having a closely-fitting, longitudinally continuous covering of a relatively non-tracking elastomeric polymeric insulating material, extending over the whole or a major part of the length of the rod or tube.
U.S. Pat. No. 3,511,698, issued May 12, 1970, to Talcott taught weatherable insulators comprising a rigid resin base member and a coating over the surface of said base member of at least ten mils thickness of cured, organopolysiloxane elastomer which comprised a silicone elastomer stock containing SiH groups and Si-alkenyl groups, and a platinum catalyst.
United Kingdom Pat. No. 1,292,276, published Oct. 11, 1972, discloses an insulator which comprises a central support of which the outer surface comprises a non-tracking electrically insulating material and at least one shed installed on the central support. The sheds are heat-shrinkable.
U.S. Pat. No. 3,965,065, issued June 22, 1976 to Elliott teaches a method of preparing an improved elastomer-forming composition which comprises forming a mixture comprising an organopolysiloxane which is convertible to the solid elastic state and aluminum hydrate, and heating the mixture at a temperature of at least 100.degree. C. for a time of at least 30 minutes.
The composition is taught to be particularly useful for the fabrication of electrical insulation having improved resistance to electric arcing and tracking.
A filler system for polymers which provides high voltage insulation is described by Penneck in U.S. Pat. No. 4,001,128, issued Jan. 4, 1977. The filler system utilizes a combination of alumina trihydrate and a chemically treated silica filler.
A rod-type insulator having improved withstand voltage characteristics under a contaminated condition is described in U.S. Pat. No. 4,174,464, issued Nov. 13, 1979. The design specifies that the leakage distance per shed divided by the pitch between the adjoining sheds is between 3.8 and 5.0 and that the leakage distance between a given point on a lower surface of one of the adjoining sheds and another given point on an upper surface of the other opposing shed divided by the distance between the given points is between 4.5 and 6.0. The lower surface of each shed has coaxial ribs forming an undulating surface.
German Pat. No. 2,650,363, issued Sept. 10, 1985 was published Nov. 17, 1977. Its U.S. equivalent is U.S. Pat. No. 4,217,466, issued Aug. 12, 1980. It teaches that the rod used in an insulator must be made from a non-saponifiable resin and the screens used must be a moisture-repellent, non-saponifiable polymer and that there must be an intermediate layer of material between the rod and the screens to protect the rod.
A process for forming open-air compound insulators is taught in U.S. Pat. No. 4,246,696, issued Jan. 27, 1981 to Bauer and Kuhl. A pre-fabricated glass fiber rod treated with silane has a rubber layer extruded over it, then after strengthening the rubber layer, prefabricated screens are bonded on by vulcanization. Their discussion of the prior art teaches the importance of the screens.
United Kingdom Pat. No. 1,601,379, published Oct. 28, 1981, teaches that for high voltage insulators, and particularly very high voltage insulators, the surface creepage distance is at least equal to 2 and preferably 3 or even higher than 3 times the straight line distance between a live conductor and ground. This is particularly true when insulators are intended for use under polluted and very polluted conditions.
Another method of assembling an insulator with sheds or screens is taught in U.S. Pat. No. 4,505,033, issued Mar. 19, 1985, to Wheeler. Sheds are either molded on or placed over a sheath of unvulcanized elastomer on a core, then the assembly is vulcanized.
In addition to the teachings represented by the above patent references, there has been a great deal of scientific literature published over the years describing insulator applications in both test applications and in commercial applications which have been closely monitored to determine the operation of the insulators. For example, Niemi and Orbeck described their concept of the properties they felt an insulation material should have and means for measuring these properties in an article, "High Surface Resistance Protective Coatings For High Voltage Insulators", presented at the IEEE Power Engineering Society Summer Meeting in San Francisco, CA, on July 9-14, 1972. They discuss a test in which an insulator is stressed at a level of 6.7 kv/in for a time period of up to 8 hours. A paper by Robert, Davis, and Dexter, presented in Russia in June of 1977 discusses the tests performed upon a silicone elastomer suggested for use in manufacturing insulators. They conclude that the silicone elastomer offers the best combination of physical, electrical, and surface properties to enable it to perform in the widest variety of environments.
High voltage polymeric insulators are normally operated under stresses in the range of from 0.25 kv/in to 0.75 kv/in as is recorded in the paper by Weihe, Macy, and Reynders, "Field Experience and Testing Of New Insulator Types in South Africa", presented at the 1980 session of the International Conference on Large High Voltage Electric Systems. A silicone insulator is described in "Silicone Elastomer Insulators For High Voltage Outdoor Applications on British Rail", a paper by Wheeler, Bradwell, Dams, and Sibbald at a BEMA Conference in May, 1982, that is a resin bonded glass fiber rod covered with a layer of silicone elastomer bonded to the rod and with end fittings adhesively bonded on each end. It appears that the insulators are 1070 mm (42.1 inches) long between end fittings and are used at 25 kv 50 Hz, giving a stress level of 0.6 kv/in.
The initial testing of new polymer based insulators indicated higher performance capability when compared to porcelain; they could operate at higher stress levels. Longer term service and test experience has shown that with aging and contamination, the polymer insulators such as are taught in the above references lose their initial voltage capability and in many cases are not able to provide the same service as porcelain and glass insulators. Current practice often recommends that polymer insulators to be used only for clean service conditions and with designed surface stresses of 0.5 to 0.7 kv/in. Operation in the field at higher stress levels has shown failure due to tracking and flashover under wet conditions. There is a need for polymer insulators capable of operating under wet, contaminated service conditions at higher stress levels exceeding 1.0 kv/in for extended periods of time.